Well I'm actually working on some changes for the next release RC2 right now, I can't remember the details of this problem but will take a detailed look at making the "Plugin Order" work properly for JoomlaComment.
EDIT: I would prefer if Daniel can decide on how to approach this once and for all, I think we need to sort this out before 4.0 becomes final. This plugin content event set to onPrepareContent still causes conflicts with other extensions, and this problem that you're having (not able to move it up or down).